Process for the on-line grafting of carboxylic acids and carboxylic acid anhydrides containing an ethylenic unsaturation onto ethylene homopolymers or copolymers and installation for the implementation of this process

ABSTRACT

(I) The (co)polymerisation of ethylene is carried out in at least one reaction zone (R) under high pressure, at 150°-320° C.; (II) the reaction product is routed into a zone (S), under an intermediate pressure P, for separation of the (co)polymer, in the molten state, from the unreacted (co)monomer(s), or the reaction product is routed into a first separation zone (S1), under an intermediate pressure (P1); the (co)polymer issuing from (S1) is then routed into a second separation zone (S2), under a pressure P2&lt;P1; (III) the (co)polymer is routed into a zone (SF), under low pressure, for final separation of the (co)polymer from the residual gases. Let-down zones (D), (D1) and (D2) are provided along the path of the (co)polymer between the zones (S) and (SF), (S1) and (S2), and (S2) and (SF) respectively, the grafting monomer or monomers being introduced on-line, in the path of the (co)polymer, at at least one point located at the level of the zone (D) or at at least one zone (D1, D2). A grafting initiator may be introduced on-line at the same time as the grafting monomer(s) flow, or separately.

This application is a continuation of application Ser. No. 07/925,923filed Aug. 7, 1992, now abandoned.

The present invention relates to a process for grafting monomers chosenfrom carboxylic acids containing an ethylenic unsaturation, thecorresponding acid anhydrides and derivatives of these acids and acidanhydrides onto ethylene homopolymers or copolymers. This process hasthe feature that the grafting monomers are injected on-line into thecontinuous production of the ethylene homopolymer or copolymer, morespecifically between the polymerisation reactor outlet and the polymerextrusion device upstream of the granulating device. The invention alsorelates to an installation for the implementation of this process.

True homopolymers and copolymers of ethylene copolymerised with themonomers indicated above are well-known; they are used, in particular,in the production of adhesives, as coatings or in the form of mixtureswith other polymers, for example polyolefins.

For some applications, however, it is desirable to have available agraft copolymer rather than a true copolymer. In fact, it is known thatthe properties of polyolefins may be modified by the grafting of acarboxylic acid or carboxylic acid anhydride containing an ethylenicunsaturation. One process for grafting unsaturated monomers ontoethylene homopolymers and copolymers of ethylene and higher C₄ -C₁₀α-olefins is described, for example, in U.S. Pat. No. 4,612,155; thus,in the case of the production of adhesives, for an identical content ofmonomers of the carboxylic acid or carboxylic acid anhydride type, theadhesiveness is better, at least in the case of relatively low contents.

The use of ethylene copolymers, grafted with a carboxylic acid orcarboxylic acid anhydride containing an ethylenic unsaturation, inlaminated films has also been disclosed, for example in European PatentApplication EP-A-0 160 984, the ethylene copolymer in this case being anethylene/alkyl acrylate copolymer.

In general, it is important when grafting monomers onto polymers toobtain a homogeneous product. If the grafting process is not conductedin a homogeneous manner, the resulting product may display variable andunacceptable properties, in particular with regard to adherence to othermaterials. Moreover, it is important to be able to graft monomers ontopolymers without giving rise to major changes in the physical propertiesof the polymers.

In order to meet this aim, it has been proposed, according to EuropeanPatent Application EP-A-0 266 994, to graft dicarboxylic acids ordicarboxylic acid anhydrides containing an ethylenic unsaturation, ortheir derivatives, onto a copolymer of ethylene with at least onemonomer chosen from vinyl alkanoates, alkyl (meth)acrylates,(meth)acrylic acid and carbon monoxide; or onto an ionomer of such acopolymer containing (meth)acrylic acid, this process consisting, ingeneral, in:

(i) forming a mixture of the copolymer, the monomer and 25-3000 ppm ofan organic peroxide having a half-life of about 1-120 minutes at 150° C.and mixing so as to obtain a uniform distribution of the monomer and ofthe peroxide in the copolymer;

(ii) malaxating the resulting mixture in an extruder at a temperaturehigher than the melting point of the said copolymer for a period of timewhich is at least four times the half-life of the organic peroxide; and

(iii) extruding the resulting graft copolymer to give a shaped article,generally consisting of pellets or other finely divided forms, which maybe used as such or as a mixture with other polymers, in particular inthe form of adhesive compositions intended for use with polymers and/ormetals, in the coextrusion of multilayer structures, in coatingcompositions, as compatibilising agents, in filled compositions and toimprove the capacity of the polymers for accepting a colouring and adye.

However, grafting of this type in an extruder has the disadvantages thatit is necessary to recover a polymer in granule form in order to remeltit, which represents a loss of energy, and that, during said recovery ofgranules, the mixture of the molten granules and the graft monomer takesplace in the presence of oxygen, which gives rise to a risk ofcrosslinking of the graft copolymer.

In RDA Pat. No. 117,077 it is indicated that a grafting reactionconducted in a device mounted immediately downstream of the reactor(moderate-pressure separator or low-pressure separator) isdisadvantageous because a large amount of homopolymer or copolymer ofthe grafting component forms, and that it is not possible to preparecopolymers having a high grafted side chain content. In order toovercome this disadvantage, it is proposed, according to said patent, tointroduce the grafting monomer either in a second zone of thepolymerisation reactor or in a high- or low-pressure receiver of thepolymerisation unit, for example a stirred reactor or an extractionmachine. However, it is indicated that this second possibility is notworthwhile if grafting monomers are used which, during graftingconducted in the polymerisation reactor, would load the recycling gasand necessitate an expensive purification of the recycling gas. Thesemonomers are maleimide, vinyl acetate or ethyl acrylate. It isrecommended to introduce maleic anhydride, which does not form part ofthis list, into the second zone of the polymerisation reactor. This isconfirmed by Example 4, which describes the production of anethylene/vinyl acetate copolymer grafted with maleic anhydride. However,the reported maleic anhydride content of 6.8% by weight probably doesnot relate solely to the grafts, some of the maleic anhydride beingcopolymerised in the form of a true copolymer with ethylene and vinylacetate.

To summarise, the injection of the grafting monomer into the reactorleads in part to a true, undesired, copolymerisation of the said monomerand injection downstream of the reactor is not recommended for maleicanhydride as grafting monomer.

The Filing Company has sought to develop a process which does not havethe abovementioned disadvantages of the prior-art techniques and whichhas the complementary advantage of being able to omit the use of devicesusing moving mechanical parts, such as the stirrers of stirred reactorsor extruder screws.

To this end, it is proposed, according to the invention, in a processfor the polymerisation or copolymerisation of ethylene under highpressure, comprising at least two separation stages downstream of thepolymerisation reactor, to introduce the grafting monomer or monomers,on-line, at the level of the let-down zone, or of at least one let-downzone, present in the installation. Thus, in the case where saidinstallation comprises a moderate-pressure separator and a low-pressureseparator and where the injection of the grafting monomer or monomers iseffected at the level of the let-down zone located between the two, itis apparent that, at the inlet of the low-pressure separator, the(co)polymer still contains about 10-15% of ethylene and, whereappropriate, unreacted comonomer(s) but that, surprisingly, despite thepresence of this high content of starting monomer(s), there is noethylene/grafting monomer copolymerisation at the inlet of the let-downvalve of the moderate-pressure separator.

Compared with grafting carried out in an extruder, the process of thepresent invention has an economic advantage because of the absence of anoperation for recovery of a granulated polymer for remelting, whichgives rise to an energy gain, and a technical advantage because theon-line mixture is free from oxygen, which eliminates the risk ofcrosslinking of the copolymer.

The present invention therefore relates to a process for the continuousproduction of an ethylene homopolymer or of a copolymer of ethylene andat least one monomer A, the said homopolymer or copolymer being modifiedby the grafting of at least one monomer B chosen from carboxylic acidscontaining an ethylenic unsaturation, their anhydrides and otherderivatives, characterised in that:

in a first step, the polymerisation of ethylene or the truecopolymerisation of ethylene and the monomer(s) A is carried out in thepresence of at least one true (co)polymerisation initiator and, whereappropriate, the customary additives, in at least one reaction zone (R)which is maintained under high pressure HP, at a temperature of between50° and 320° C.;

in a second step,

either the reaction product is routed into a separation zone (S),maintained under an intermediate pressure P, in which the (co)polymerformed is separated, in the molten state, from the unreacted(co)monomer(s);

or the reaction product is routed into a first separation zone (S1),which is maintained under an intermediate pressure P1, in which the(co)polymer formed is separated, in the molten state, from the unreacted(co)monomer(s); then the (co)polymer obtained is routed into a secondseparation zone (S2), maintained under an intermediate pressure P2<P1,in which zone the (co)polymer in the molten state, issuing from zone(S1), is separated from the unreacted (co)monomer(s); and

in a third step, the (co)polymer obtained is routed into a finalseparation zone (SF), maintained under a low pressure LP, in which zonethe (co)polymer is separated from the residual gases, appropriatelet-down zones (D), (D1) and (D2) being provided along the path of the(co)polymer between the zones (S) and (SF), (S1) and (S2), and (S2) and(SF) respectively, the monomer(s) B, if appropriate dissolved in asolvent or suspended in a dispersing medium, being introduced on-line,into the path of the (co)polymer, at at least one point located at thelevel of the let-down zone (D) or at at least one point located at thelevel of at least one let-down zone (D1, D2), grafting moreover beingcarried out, if necessary, in the presence of at least one appropriateinitiator.

According to a preferred embodiment of the process of the presentinvention, the flow consisting of or comprising the monomer(s) B isinjected upstream of the let-down zone in question (D; D1; D2), betweenthe associated let-down point and the outlet of the separation zonelocated just upstream of the said let-down point. In the case where thelet-down zone comprises a needle valve, the let-down point can beconsidered as corresponding to the region of the needle; for reasons oftechnical assembly, the inlet of the flow of grafting monomer(s) (B)will take place at least a short distance from the said needle.

According to another embodiment of the process according to theinvention, the flow consisting of or comprising the monomer(s) B isinjected downstream of the let-down zone in question (D; D1; D2), in thevicinity of the associated let-down point. The latter expression isunderstood to mean that the inlet of the flow of grafting monomer(s)takes place in a region relatively close to the let-down point inquestion.

According to the invention, the first step is advantageously carried outunder a pressure HP of about 400 to 3000 bar, the second step under apressure P of about 50 to 500 bar or under a pressure P1 of about 100 to500 bar and then under a pressure P2 of about 10 to 70 bar, and thethird step under a pressure LP of about 0.5 to 10 bar, on condition thatthe value of LP is lower than the value of P2.

Moreover, the second and the third steps are carried out at atemperature which is advantageously between 150° and 300° C.

According to the invention, the following are used, in particular, asmonomer(s) A: a C₃ -C₁₀ alpha-olefin, for example propylene, but-1-ene,4-methylpent-1-ene, hex-1-ene or oct-1-ene, allowing the production ofethylene copolymers by the Ziegler route, or a vinyl alkanoate, such asvinyl acetate; a C₁ -C₁₂ alkyl (meth)acrylate, such as ethyl acrylate,methyl acrylate, butyl acrylate or methyl methacrylate; (meth)acrylicacid; and carbon monoxide, allowing the production of polar ethylenecopolymers by the free radical route.

The (co)polymerisation initiator is a free radical initiator, such as aperoxide, a perester or a hydroperoxide, in the free radical(co)polymerisation of ethylene; or a catalyst system of the Ziegler typecomprising, in particular, at least one transition metal halide and anorganometallic activator, in the case of the ionic (co)polymerisation ofethylene/α-olefin.

The following are advantageously used as monomer(s) B: (meth)acrylicacid, maleic acid, fumaric acid, itaconic acid, crotonic acid, itaconicanhydride, maleic anhydride or a substituted maleic anhydride, such asdimethylmaleic anhydride, or a salt, amide, imide or ester of acarboxylic acid containing an ethylenic unsaturation, such as monosodiumand disodium maleate, acrylamide, maleimide and diethyl fumarate; maleicanhydride is used in particular.

Preferably, the monomer(s) B is or are introduced in the form of asolution in a solvent chosen from ethylene carbonate, propylenecarbonate, diethyl carbonate and the esters of fatty acids and heavyalcohols, such as the butyl, hexyl and octyl acetates, or in the form ofa dispersion in a dispersing medium, such as polyhexene or a polyolefin,in particular polyethylene, wax or fat, optionally with at least onesolvent. The monomer(s) B/solvent weight ratio is advantageously between0.5/1 and 1.5/1, preferably the highest possible concentration permittedby the solvent power of the solvent chosen and the temperature. Themonomer(s) B/dispersing medium weight ratio is advantageously between1/1 and 4/1. Moreover, the flow consisting of or comprising themonomer(s) B is heated so as to facilitate its on-line injection at atemperature which is adequate but below that which would give rise tothe decomposition of the initiator which may be used conjointly. It ispossible to use a temperature of between, for example, 100° and 140° C.for said flow.

The following are advantageously used as grafting initiator: a peroxideand/or a perester, such as dicumyl peroxide or di-tert-butyl peroxide,tert-butyl perbenzoate or 2,5-dimethyl-2,5-(di-tert-butylperoxy)-hexaneor a diazo compound.

The grafting initiator(s) is or are introduced into the flow of themonomer(s) B injected into the production line, and/or separately, alsoon-line, in particular upstream or advantageously downstream of theinjection point or of at least the first injection point for thegrafting monomer(s) B. In the latter case, these initiators aredissolved or dispersed in an appropriate medium, which may be a polymermedium.

According to the present invention it is also possible to carry out thegrafting of at least one monomer B as defined above onto a copolymer ofethylene and at least one alpha-olefin, which copolymer is prepared inthe presence of a catalyst system of the Ziegler type and modified bybringing the copolymer thus obtained into contact with at least one freeradical initiator, in particular in an amount of 0.01 to 1 millimol perkg of the said copolymer. Free radical intiators of this type may bechosen from:

peroxy compounds such as peroxides (for example, di-tert-butylperoxide), peresters and hydroperoxides;

benzopinacol;

diazo compounds, such as 2,2'-azo-bis(acyloxyalkanes), such as2,2'-azo-bis(acetoxybutane), 2,2'-azo-bis(acetoxyisobutane),2,2'-azo-bis(propionoxypropane),2,2'-azo-bis(2-acetoxy-4-methylpentane), or1,1'-azo-bis(1-formyloxycyclohexane); and

hydrocarbons possessing labile carbon-carbon bonds, such as, forexample, diaryldialkylalkanes, such as 2,3-diphenyl-2,3-dimethylbutane(commonly known by the name biscumyl) or3,4-diphenyl-3,4-dimethylhexane.

Details relating to said polymers modified in this way, the process forobtaining them, the choice of free radical initiators and the time forwhich the copolymer is brought into contact with the latter, etc., willbe found in European Patent Applications EP-A-0 083 521 and EP-A-0 231699, which are incorporated here by reference. The copolymers ofethylene and at least one α-olefin having from 3 to 12 carbon atoms,comprising 0.5 to 10 mol % of units derived from the said α-olefin andmodified according to EP-A-0 083 521, are characterised in that theirmeasured limiting viscosity is between 1.5 and 10 times their limitingviscosity calculated from the molecular mass distribution; with regardto the polymer compositions according to EP-A-0 231 699, these contain:a) from 90 to 99.8% by weight of at least one modified ethylene polymerand, where appropriate, at least one C₃ -C₁₂ α-olefin, the said polymercontaining at least 80 mol % of units derived from ethylene, and b) from0.2 to 10% by weight of at least one polymer phase comprising at leastone polymer containing at least 90 mol % of units derived from at leastone C₃ -C₁₂ α-olefin and at most 10 mol % of units derived fromethylene, the said compositions having a measured limiting viscosity ofbetween 1.5 and 100 times their limiting viscosity calculated from themolecular mass distribution.

Moreover, the free radical initiator(s) introduced with a view to saidmodification may be in the form of a solution in a solvent or in theform of a suspension in a dispersing medium of the types indicatedabove, or in a polymer phase consisting, in particular, of ethyleneand/or the polymerised comonomer A (by way of example, a polymer phasecomprising at least 90 mol % of units derived from at least one C₃ -C₁₂α-olefin and at most 10 mol % of units derived from ethylene isdescribed in EP-A-0 231 699).

Within the framework of the present invention, these initiators areintroduced on-line, into the path of the (co)polymer, at at least onepoint located between the final reaction zone(s) of a multi-zonereactor, or in the final reactor of a reactor assembly connected inseries, and the outlet of the conversion zone, which generally followsthe final separation zone and in which an extrusion followed by agranulation of the intended (co)polymer are carried out. It is alsopossible to incorporate these free radical initiators by malaxating withthe granules of the final graft (co)polymer.

Moreover, a grafting initiator(s)/monomer(s) B weight ratio which may beup to 1 and is preferably between 0 and 0.5 and an injected monomer(s) Bcontent by weight of 0.05 to 2 parts per 100 parts of the copolymer, andpreferably of 0.1 to 1.5 parts, are used.

According to the invention, the monomer(s) A is or are used in an amountof, in particular, from 0.1 to 90 mol % with respect to the sum ofethylene+monomer(s) A, for example 0.1 to 10 mol % during the productionof the polar copolymers, and for example 2 to 90 mol % during theproduction of copolymers of ethylene and α-olefin(s).

The present invention also relates to an installation for theimplementation of the process as defined above, characterised in that itcomprises, in series:

at least one autoclave or tubular polymerisation reactor (R) under highpressure;

a separator (S) capable of operating under an intermediate pressure P,or two separators (S1; S2) in series, capable of operating underintermediate pressures P1 and P2 respectively, where P2<P1;

a low-pressure final separator (SF);

an extruder; and

a granulating device,

(co)polymer lines connecting the reactor (R) and the separators (S; S1;S2; SF), let-down valves, in particular needle valves, being arrangedrespectively on the outlets of the reactor (R) and the separators (S;S1; S2) separating the unreacted (co)monomer(s) from the (co)polymerformed; at least one line for injection of the flow consisting of orcomprising the grafting monomer(s) B and, where appropriate, at leastone grafting initiator opening into the line, in the let-down zone or alet-down zone containing the associated let-down valve; whereappropriate, at least one-line for the separate on-line injection of thegrafting initiator(s), preferably downstream of the injection point, orof at least the first injection point, for the grafting monomer(s) B;where appropriate, at least one line for the separate on-line injectionof at least one free radical initiator with a view to modification of anethylene/α-olefin copolymer prepared by Ziegler catalysis in thereaction zone, at at least one point located between the final reactionzone(s) of a multi-zone reactor or in the final reactor of a reactorassembly connected in series and the outlet of the granulating device;pumping means being provided on the line(s) for the injection of thegrafting momoner(s) flow and on the line(s) for the separate injection,if this is provided, of the grafting initiator(s) and/or of the freeradical initiator(s) with a view to the abovementioned modification.

The graft copolymers obtained in accordance with the present inventionfind application in particular for formulating injection mouldingcompositions, for forming films which can be used in the hygiene,agriculture and refrigeration sectors, etc.

In order better to illustrate the subject of the present invention,several illustrative embodiments will be described below, with referenceto the appended drawings, in which:

FIGS. 1 and 2 each show, schematically, an installation for theimplementation of the process according to the present invention; and

FIG. 3 is a diagram of a needle valve for effecting each let-down in theabovementioned installations, showing a possible assembly of the linefor the injection of the grafting monomer(s).

If reference is made to FIG. 1, it is seen that is represented aproduction unit for an ethylene graft polymer or copolymer.

The autoclave reactor in which the polymerisation of ethylene or thecopolymerisation of ethylene with at least one monomer A takes place isdenoted in its entirety by R; it consists of an elongated recipient,arranged vertically, assembled in three superimposed zones, whichoperates under high pressure HP. The first two zones R1 and R2 areprovided with a concentric injector and the third zone R3 is providedwith a simple injector. Temperature control is effected at three pointsr1, r2 and r3, the temperature being taken at points T1, T2 and T3. Thegaseous ethylene/monomer(s) A mixture, containing, where appropriate, atransfer agent such as hydrogen, which is used in an amount of, inparticular, up to 2% by volume, so as to control the fluidity index ofthe (co)polymer obtained, is injected at points t, m1 and m2. Anappropriate polymerisation initiator is injected at i1, i2 and i3, in amanner known per se and so as to maintain the copolymerisationtemperature at the chosen value.

The (co)polymer leaving the reaction zone R is fed, via a line 1, to aseparation device S, consisting of a vertically arranged elongatedvessel maintained under an intermediate pressure P, a let-down needlevalve 2 being arranged on the line 1.

In the separator S the unreacted monomers are removed from the reactionmixture and these unreacted monomers leave the upper part via the line 3and are recycled in a known manner, for example to the inlet of ahypercompressor mounted between the compressor which has to compress thefeed charge and the reactor R, as described in Canadian Patent CA-A-1161 196.

In the same way as above, the flow rich in (co)polymer leaving theseparation zone S is fed, via a line 4, to the final separation deviceSF, which is of a hopper type and operates under a low pressure LP. Aletdown needle valve 5 is also inserted in line 4. The injection, viathe line 6, of a solution contained in a reservoir r, whichadvantageously is heated, of grafting monomer(s) B/graftinginitiator/solvent, via a high-pressure pump 7 (FIG. 3) is provided at apoint located upstream of the needle of said let-down valve 5. The pumpchosen as pump 7 may be an alternating metering pump with continuousflow control, or a geared pump.

The residual gases leaving via the line 8 of the low-pressure hopper SFmay be recycled by being fed to the compressor (not shown); with regardto the mixture 9, this is fed to a screw extruder, which it leaves inthe form of a rod, which is then fed, in known manner, to a granulatingdevice, which is not shown.

EXAMPLES 1 to 21

General operating method:

The installation shown in FIG. 1 was used.

The operating conditions were as follows:

pressure HP of the reactor R: that indicated in Table 1;

monomer injection: as indicated in the above description of theinstallation with the following distribution:

t: 1/3; m1: 1/3; m2: 1/3,

injection of the initiator, consisting of tert-butyl peroctanoate forExamples 1 to 17, di-tert-butyl peroxide for Examples 18 to 20 and amixture of tert-butyl perbenzoate (60%) and tert-butyl perneodecanoate(40%) for Example 21: as indicated in the above description of theinstallation, with the following distribution:

i1: 60%; i2: 20%; i3: 20%,

temperature profile:

T1: 250° C. (Examples 18 to 20) or 170° C. (other examples)

T2: 260° C. (Examples 18 to 20) or 190° C. (other examples); and

T3: 275° C. (Examples 18 to 20) or 215° C. (other examples).

total flow rate of the gaseous monomers fed to the reactor: 200 kg/h;

percentage of monomer A as mol % with respect to the sum ofethylene+monomer A;

prevailing pressure in the separator S, P=250 bar.

A 250 g/l solution of maleic anhydride (monomer B) in propylenecarbonate (Examples 1 to 11) or in diethyl carbonate (Examples 12 to 21)was injected via the line 6 at a flow rate so as to comply with thepercentage by weight of maleic anhydride, injected with respect to the(co)polymer, indicated in Table 1. An initiator, also as indicated inTable 1, was present in the solution (except for Examples 1, 6, 12, 18and 21). The initiator/maleic anhydride weight ratio is also indicatedin Table 1.

Table 1 also indicates the monomer A content of the copolymer obtained(in % by weight), the fluidity index of the graft (co)polymer obtained,determined in accordance with the standard ASTM D1238, condition E, theamount of maleic anhydride grafted (expressed in ppm) and the degree ofgrafting.

The determination of the maleic anhydride grafted was carried out on theabsorption at 1785 cm⁻¹ of the anti-symmetric valency vibration of thecarbonyl groups, taking as a basis a calibration carried out usingethylene/maleic anhydride copolymers and ethylene/n-butylacrylate/maleic anhydride terpolymers produced by free radicalcopolymerisation. The propylene carbonate (or the diethyl carbonate) andthe maleic anhydride and the other volatile materials were extractedwith acetone for 24 hours; this determination by infrared spectroscopywas carried out on a film having a constant thickness of 50 μm. Thedegree of grafting represents the ratio of the amount of maleicanhydride grafted onto the (co)copolymer to the amount of maleicanhydride injected on-line, with respect to the same amount of(co)polymer.

                                      TABLE 1                                     __________________________________________________________________________                                        % by weight                                                    % by weight                                                                          Ratio by weight                                                                       of monomer    Maleic                                           of maleic                                                                            of initiator/                                                                         A in the      anhydride                   HP       Monomer A                                                                             Initia-                                                                           anhydride                                                                            maleic  final  FI     grafted                                                                             Degree of             Example                                                                            (bars)                                                                            Nature                                                                            mol %                                                                             tor I                                                                             injected                                                                             anhydride                                                                             copolymer                                                                            (g/10 min)                                                                           (ppm) grafting              __________________________________________________________________________     1   2050                                                                              BuA 0.60                                                                              --  0.19   0       14.5   2.14   360   0.19                   2   "   "   0.58                                                                              I1  0.18   0.05    14     1.85   470   0.26                   3   "   "   0.58                                                                              I2  0.21   0.05    14     1.85   405   0.186                  4   "   "   0.62                                                                              I1  0.42   0.05    15     3.5    625   0.148                  5   "   "   0.62                                                                              I2  0.17   0.2     15     2.2    440   0.261                  6   "   "   0.59                                                                              --  0.5    0       14.4   1.81   480   0.096                  7   "   "   0.58                                                                              I1  0.53   0.2     14     2.8    645   0.12                   8   "   "   0.58                                                                              I1  0.18   0.2     14     2.55   425   0.233                  9   "   "   0.62                                                                              I3  0.19   0.2     15     2.31   500   0.264                 10   "   "   0.58                                                                              I3  0.54   0.05    14     2.07   400   0.08                  11   "   "   0.62                                                                              I2  0.18   0.5     15     2.1    1000  0.55                  12   "   "   0.63                                                                              --  0.46   0       15.3   2.2    780   0.17                  13   "   "   0.63                                                                              I2  0.4    0.5     15.3   1.8    1850  0.46                  14   1960                                                                              "   0.60                                                                              I4  0.5    0.05    14.7   1.4    1700  0.34                  15   "   "   0.62                                                                              I4  0.5    0.2     15.1   1.4    1900  0.38                  16   "   "   0.62                                                                              I4  0.5    0.5     15     1.8    2050  0.41                  17   1600                                                                              "   0.42                                                                              I4  1      0.2     10.1   5.5    4200  0.42                   18* 1500                                                                              --  0   --  0.4    0       0      8      2100  0.53                   19* "   --  0   I4  0.4    0.05    0      8      2200  0.55                   20* "   --  0   I4  0.4    0.5     0      8      2400  0.60                  21   1720                                                                              VA  1.45                                                                              --  0.4    0       5.1    2.3    1720  0.43                  __________________________________________________________________________     *Free radical ethylene homopolymer                                            BuA: Butyl acrylate                                                           VA: Vinyl acetate                                                             I1: Dicumyl peroxide                                                          I2: Tertbutyl perbenzoate                                                     I3: Ditert-butyl peroxide                                                     I4: 2,5Dimethyl-2,5-di(tert-butylperoxy)-hexane                          

If reference is now made to FIG. 2, it can be seen that this shows aninstallation for the production of graft ethylene copolymers analogousto that in FIG. 1, except that it is provided with a second separationzone S2. The two intermediate separation zones S1 and S2 operate at,respectively, P1 and P2, P1 being higher than P2. The lines associatedwith the second separator S2 are denoted by reference numerals greaterthan 10.

It is possible to inject the monomer B+solvent+initiator mixture, orelse just upstream of the needle of the valve 5, as shown by the solidline on FIG. 2. Similarly, it is also possible to inject the mixturebased on monomer B via line 16, as shown by a broken line in FIG. 2. Itis also possible to inject the mixture based on monomer B via line 16only.

EXAMPLE 22

In the installation shown in FIG. 1, a gaseous mixture comprising 50% byweight of ethylene and 50% by weight of but-1-ene, introduced into thereactor R at point t, was copolymerised continuously. 0.03% by volume ofhydrogen with respect to the gaseous mixture was also introduced at thesame point.

The copolymerisation catalyst, comprising the co-ground compound TiCl₃,1/3AlCl₃ and 2.5MgCl₂ in suspension in a C₁₀ -C₁₂ saturated hydrocarboncut, was introduced at point i1. It was activated by means oftriethylaluminium using an Al/Ti ratio of 10.

The three zones of the reactor were kept at a temperature of 250° C.under a pressure of 800 bar.

The separator S was kept under a pressure of 250 bar; the low-pressurehopper SF was kept under a pressure of 10 bar.

A solution containing, per liter, 250 g of maleic anhydride and 50 g of2,2-azo-bis(acetoxypropane) in diethyl carbonate was introduced via line(6) by means of the pump (7). The flow rate of this solution was suchthat 0.15 g of maleic anhydride were introduced per 100 g of copolymeroriginating from the separator S. The residual gases issuing from thelow-pressure hopper SF were not recycled into the polymerisationinstallation.

A copolymer of ethylene and but-1-ene grafted with 0.05% by weight ofmaleic anhydride and having a density of 0.923 and a fluidity index FI(determined in accordance with the standard ASTM D-1238, condition L) of3.7 g/10 min was obtained.

By way of comparison, if the solution of maleic anhydride and initiatoris not introduced, a copolymer of ethylene and but-1-ene which has thesame density and an FI of 4.5 g/10 min is obtained.

We claim:
 1. A process for the continuous production of an ethylenehomopolymer, or a copolymer of ethylene and at least one comonomer A,the homopolymer or copolymer being modified by grafting thereto at leastone monomer B selected from carboxylic acids containing an ethylenicunsaturation, their anhydrides and other derivatives, said processcomprising:in a first step, polymerizing ethylene, or copolymerizingethylene and at least one comonomer A in the presence of at least oneinitiator in at least one reaction zone (R) under high pressure (HP) ata temperature between 150° and 320° C.; in a second step, passing thereaction product into a separation zone (S), maintained under anintermediate pressure (P), wherein the polymer or copolymer formed isseparated, in the molten state, from the unreacted monomer orcomonomers; and in a third step, passing the polymer or copolymerobtained into a final separation zone (SF), maintained under a lowpressure (LP), wherein the polymer or copolymer is separated from theresidual gases, wherein a letdown zone (D) is provided along the path ofthe polymer or copolymer between the zones (S) and (SF), and wherein atleast one monomer B is grafted to the polymer or copolymer byintroducing said at least one monomer B into the path of the polymer orcopolymer upstream of letdown zone (D) between separator zone (S) andletdown zone (D), and wherein said first step is carried out under apressure (HP) ranging from 400 to 3000 bar, said second step under apressure (P) ranging from 50 to 500 bar, and said third pressure under apressure (LP) ranging from 0.5 to 10 bar.
 2. The process according toclaim 1, wherein the second and third steps are carried out at atemperature between 150° and 300° C.
 3. The process according to claim1, wherein said comonomer A is selected from the group consisting of C₃-C₁₀ alpha-olefins, allowing the production of ethylene copolymers bythe Ziegler route, or is selected from the group consisting of a vinylalkanoate, a C₁ -C₁₂ alkyl (meth)acrylate, (meth)acrylic acid and carbonmonoxide, allowing the production of polar ethylene copolymers by thefree radical route.
 4. The process according to claim 1, wherein said atleast one monomer B is selected from the group consisting of(meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, crotonicacid, itaconic anhydride, maleic anhydride, a substituted maleicanhydride, and a salt, amide, imide or ester of a carboxylic acidcontaining an ethylenic unsaturation.
 5. The process according to claim1, wherein said at least one monomer B is introduced in the form of asolution in at least one solvent selected from the group consisting ofethylene carbonate, propylene carbonate, diethyl carbonate, the estersof fatty acids,. and the esters of heavy alcohols, or in the form of adispersion in a dispersing medium, the monomer(s) B/solvent weight ratioranging between 0.5/1 and 1.5/1 and the monomer(s) B/dispersing mediumweight ratio ranging between 1/1 and 4/1.
 6. The process according toclaim 5, wherein said at least one monomer B has been heated prior tointroduction to a temperature below the decomposition temperature of thegrafting initiator.
 7. The process according to claim 1, wherein thegrafting initiator is selected from the group consisting of a peroxide,a perester, and a diazo compound.
 8. The process according to claim 1,wherein the grafting of at least one monomer B is carried out onto acopolymer of ethylene and at least one alpha-olefin, which copolymer isprepared in the presence of a catalyst system of the Ziegler type andmodified by bringing the copolymer thus obtained into contact with atleast one free radical initiator, said at least one free radicalinitiator being introduced on-line, into the path of the copolymer, atat least one point located between the final reaction zone of amultizone reactor, or in the final reactor of a reactor assemblyconnected in series, and the outlet of the zone for conversion byextrusion and granulation, which follows the final separation zone. 9.The process according to claim 1, wherein the graftinginitiator(s)/monomer(s) B weight ratio is less than 1 and said at leastone monomer B is introduced in an amount by weight ranging from 0.05 to2 parts per 100 parts of the copolymer.
 10. The process according toclaim 3, wherein said at least one comonomer A is used in an amountranging from 0.1 to 10 mol % of the sum of ethylene and monomer(s) Aduring the production of the polar copolymers, and an amount rangingfrom 2 to 90 mol % of the sum of ethylene and monomer(s) A is usedduring the production of copolymers of ethylene and alpha-olefins. 11.The process according to claim 4, wherein said at least one monomer B isselected from the group consisting of dimethylmaleic anhydride,monosodium maleate, disodium maleate, acrylamide, maleimide and diethylfumarate.
 12. The process according to claim 4, wherein said at leastone monomer B is maleic anhydride.
 13. The process according to claim 5,wherein the dispersion medium is a polyethylene wax or a fat.
 14. Theprocess according to claim 7, wherein the grafting initiator is selectedfrom the group consisting of dicumyl peroxide, di-tert-butyl peroxide,tert-butyl perbenzoate and 2,5-dimethyl-2,5-(di-tert-butylperoxy)-hexane.
 15. The process according to claim 9,wherein the grafting initiator(s)/monomer(s) B weight ratio is between 0and 0.5 and the monomer(s) B content by weight is 0.1 to 1.5 parts per100 parts of copolymer.
 16. The process according to claim 1, whereinsaid grafting is carried out by adding at least one grafting initiatorto said at least one monomer B prior to introduction of said at leastone monomer B into the path of the polymer or copolymer.
 17. The processaccording to claim 1, wherein said grafting is carried out byintroducing at least one grafting initiator upstream of the introductionpoint of said at least one monomer (B).
 18. A process for the continuousproduction of an ethylene homopolymer, or a copolymer of ethylene and atleast one comonomer A, the homopolymer or copolymer being modified bygrafting thereto at least one monomer B selected from carboxylic acidscontaining an ethylenic unsaturation, their anhydrides and otherderivatives, said process comprising:in a first step, polymerizingethylene, or copolymerizing ethylene and at least one comonomer A in thepresence of at least one initiator in at least one reaction zone (R)under high pressure (HP) at a temperature between 150° and 320° C. in asecond step, passing the reaction product into a first separation zone(S1), maintained under an intermediate pressure (P1), wherein thepolymer or copolymer formed is separated, in the molten state, from theunreacted monomer or comonomers; then passing the polymer or copolymerinto a second separation zone (S2), maintained under an intermediatepressure (P2) that is lower than (P1), wherein the polymer or copolymeris further separated, in the molten state, from the unreacted monomer orcomonomers; in a third step, passing the polymer or copolymer obtainedinto a final separation zone (SF), maintained under a low pressure (LP),wherein the polymer or copolymer is separated from the residual gases,wherein a letdown zone (D1) is provided between zones (S) and (S2), anda letdown zone (D2) is provided between zones (S2) and (SF), and whereinat least one monomer B is grafted to the polymer or copolymer byintroducing said at least one monomer B into the path of the polymer orcopolymer upstream of at least one of letdown zones (D1) or (D2), andwherein said first step is carried out under a pressure (HP) rangingfrom 400 to 3000 bar, said second step under a pressure (P1) rangingfrom 100 to 500 bar and a pressure (P2) ranging from 10 to 70 bar, andsaid third step under a pressure (LP) ranging from 0.5 to 10 bar,wherein the pressure (LP) is lower than the pressure (P2).
 19. Theprocess according to claim 18, wherein the second and third steps arecarried out at a temperature between 150° and 300° C.
 20. The processaccording to claim 18, wherein said comonomer A is selected from thegroup consisting of C₃ -C₁₀ alpha-olefins, allowing the production ofethylene copolymers by the Ziegler route, or is selected from the groupconsisting of a vinyl alkanoate, a C₁ -C₁₂ alkyl (meth)acrylate,(meth)acrylic acid, and carbon monoxide, allowing the production ofpolar ethylene copolymers by the free radical route.
 21. The processaccording to claim 18, wherein said at least one monomer B is selectedfrom the group consisting of (meth)acrylic acid, maleic acid, fumaricacid, itaconic acid, crotonic acid, itaconic anhydride, maleicanhydride, a substituted maleic anhydride, and a salt, amide, imide orester of a carboxylic acid containing an ethylenic unsaturation.
 22. Theprocess according to claim 18, wherein said at least one monomer B isintroduced in the form of a solution in at least one solvent selectedfrom the group consisting of ethylene carbonate, propylene carbonate,diethyl carbonate, the esters of fatty acids, and the esters of heavyalcohols, or in the form of a dispersion in a dispersing medium, themonomer(s) B/solvent weight ratio ranging between 0.5/1 and 1.5/1 andthe monomer(s) B/dispersing medium weight ratio ranging between 1/1 and4/1.
 23. The process according to claim 22, wherein said at least onemonomer B has been heated prior to introduction to a temperature belowthe decomposition temperature of the grafting initiator.
 24. The processaccording to claim 18, wherein the grafting initiator is selected fromthe group consisting of a peroxide, a perester, and a diazo compound.25. The process according to claim 18, wherein the grafting of at leastone monomer B is carried out onto a copolymer of ethylene and at leastone alpha-olefin, which copolymer is prepared in the presence of acatalyst system of the Ziegler type and modified by bringing thecopolymer thus obtained into contact with at least one free radicalinitiator, said at least one free radical initiator being introducedon-line, into the path of the copolymer, at at least one point locatedbetween the final reaction zone of a multizone reactor, or in the finalreactor of a reactor assembly connected in series, and the outlet of thezone for conversion by extrusion and granulation, which follows thefinal separation zone.
 26. The process according to claim 18, whereinthe grafting initiator(s)/monomer(s) B weight ratio is less than 1 andsaid at least one monomer B is introduced in an amount by weight rangingfrom 0.05 to 2 parts per 100 parts of the copolymer.
 27. The processaccording to claim 20, wherein said at least one comonomer A is used inan amount ranging from 0.1 to 10 mol % of the sum of ethylene andmonomer(s) A during the production of the polar copolymers, and anamount ranging from 2 to 90 mol % of the sum of ethylene and monomer(s)A is used during the production of copolymers of ethylene andalpha-olefins.
 28. The process according to claim 21, wherein said atleast one monomer B is selected from the group consisting ofdimethylmaleic anhydride, monosodium maleate, disodium maleate,acrylamide, maleimide and diethyl fumarate.
 29. The process according toclaim 21, wherein said at least one monomer B is maleic anhydride. 30.The process according to claim 22, wherein the dispersion medium is apolyethylene wax or a fat.
 31. The process according to claim 24,wherein the grafting initiator is selected from the group consisting ofdicumyl peroxide, di-tert-butyl peroxide, tert-butyl perbenzoate and2,5-dimethyl-2, 5-(di-tert-butylperoxy)-hexane.
 32. The processaccording to claim 26, wherein the grafting initiator(s)/monomer(s) Bweight ratio is between 0 and 0.5 and the monomer(s) B content by weightis 0.1 to 1.5 parts per 100 parts of copolymer.
 33. The processaccording to claim 18, wherein said grafting is carried out by adding atleast one grafting initiator to said at least one monomer B prior tointroduction of said at least one monomer B into the path of the polymeror copolymer.
 34. The process according to claim 18, wherein saidgrafting is carried out by introducing at least one grafting initiatorupstream of the introduction point of said at least one monomer (B).